1. Field of the Invention
This invention relates to blood collection and, more particularly, relates to a blood sample collection and separation assembly and method for its use.
2. Background
Blood samples are routinely taken in evacuated tubes. One end of a double-ended needle is inserted into a patient's vein. The other end of the needle then punctures a stopper covering the open end of the tube so that the vacuum in the tube draws the blood sample through the needle into the tube. Using this technique, a plurality of samples can be taken using a single needle puncture of the skin.
Prior to clinical examination, blood collected in evacuated tubes often must be separated into a liquid fraction of plasma or serum and a solid fraction of packed or clotted cells. It is conventional in the art to perform this separation by centrifugation. A problem which often arises is remixing of the solid and liquid phases, after centrifugation, when the tube is handled, particularly when physical separation of the two phases is attempted.
Thus, it is also conventional in the art to provide some means to maintain the separation of the solid and liquid phases after the centrifugation is complete. Practically all modern blood collection and serum separation devices use a thixotropic gel as the phase separator. These devices rely on the specific gravity of the gel to position the gel at the solid-liquid interface. The gel is initially placed in the bottom of the tube so that the gel moves upwardly to reach the interface. Use of a thixotropic gel as phase separator is disclosed in U.S. Pat. No. 4,050,451 to Colombus.
Problems with thrixotropic gels are related to narrow formulation specifications required to maintain correct thixotropic behavior and specific gravity which greatly complicate high-volume manufacturing. Slow changes in the gel formulation with aging causes product performance problems and limits shelf life. Also, constituents of the gel in the form of gel particles can significantly contaminate serum or plasma and affect downstream chemistry analysis or chemistry analyzing instruments. The present invention is directed to overcoming these disadvantages.